1. Field of the Invention
The invention relates generally to electronic materials processing, and more particularly to a system and method for fabricating microlenses on Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Related Art
Unlike edge-emitting lasers that emit laser light horizontally from the etched side edge of a semiconductor stack, VCSELs are characterized in that the emitted laser beam is emitted vertically from the substrate surface. VCSELs thus have significant advantages over edge-emitting lasers in the areas of lower manufacturing, packaging, alignment, and testing costs, as well as lower power dissipation.
FIG. 8 is a perspective view depicting a multiple beam laser scanner or ROS (raster output scanner) system 800 used, for example, in a high resolution, high speed printing apparatus. System 800 generally utilizes a two-dimensional VCSEL array 810 that transmits several light beams 815 through pre-polygon optical devices 820 to a rotating polygonal mirror 830, which scans the beams through scan optics 840 and a directing mirror 850 to a photo-receptor 860, which performs high speed printing/scanning functions in response to the modulated intensity of the individual beams according to known techniques.
FIG. 9 is an enlarged plan view showing a thirty-six beam VCSEL array 810A, which represents one type of VCSEL array utilized in systems such as those depicted in FIG. 8. Each VCSEL 812 of array 810A is formed by an active region (e.g., GaAs) surrounded by an electrode (e.g., gold). In each VCSEL, laser photons resonate between mirrors grown into the substrate structure, and then emit vertically from light-emitting regions of the wafer surface.
Referring again to FIG. 8, in order for system 800 to operate as intended, the beams generated by array 810 must have sufficient energy to adequately expose the photoreceptor 860 or recording medium. That is if the light beams are too low in energy or power, then they will be unable to generate an image with enough light intensity that can be detected, captured, or recorded by the photoreceptor or recording medium.
One approach to addressing this problem is to increase the intensity generated by each beam, and to increase the sensitivity of the photoreceptor, thereby providing a suitable amount of light exposure. However, the beam intensity of current VCSEL devices is limited, and driving the VCSELs harder with more current can adversely affect lifetime and single transverse mode emission characteristics.
Another approach to improving the throughput of the optical system without changing the spacing between VCSELs is to utilize microlenses to reduce the divergence angle of the individual VCSELs in the array. This approach allows more light to be captured by the optical system and transmitted to the photoreceptor.
Current approaches to integrate microlenses and VCSEL arrays for this type of purpose include the hybrid mechanical assembly of a VCSEL array and a separate microlens array, and forming microlenses on the VCSEL by deposition and reflow of material like photoresist as additional steps in the VCSEL array fabrication process. A problem with the first conventional approach is that aligning the separate microlens array with the VCSEL array is time consuming and tedious, and prone to alignment error that can greatly reduce the effectiveness of the lens array. A problem with the second conventional approach is that the additional is that the additional processing steps significantly increase fabrication costs.
What is needed is an efficient and reliable method for forming microlenses on VCSEL arrays that avoids the problems associated with the conventional approaches discussed above.